Optical proximity sensors may be used in electronic devices such as for example smart phones. An optical proximity sensor may be implemented as active sensor arrangement being able to detect the presence of a nearby object without physical contact. An emitter may emit electromagnetic radiation, for example infrared radiation, and a detector may detect radiation returning after being reflected by an object.
Since the emitter and the detector may be positioned adjacent to each other and the radiation may have to pass through a cover, for example a cover glass or a cover plastic with ink printed on it, radiation may get reflected by the cover or ink, absorbed by the cover or ink, or scattered by the cover or ink and consequently be detected without having left the sensor arrangement and reached an object. This phenomenon is denoted as optical crosstalk and may lead to a reduced signal-to-noise ratio. A required or desired signal-to-noise ratio may thus impose limits on the intensity of the emitted radiation and/or the sensitivity of the detector to limit crosstalk. Consequently, a detection range, in particular a maximum detection distance, of the sensor arrangement may be reduced.
Optical crosstalk and a reduced detection range may be particularly problematic in arrangements where the ink layer covers the emitter and/or the detector to disguise them from an external observer.